Using old hardwood with new hardwood

My husband and I were given about 400 sq.ft. of hardwood to use in our new home. We'll need about 300 sq.ft. more to finish the project, but my concern is in using the old hardwood with new hardwood that we'll have to purchase. We went to a flooring store, and the fit is not super tight (you can slide it in and out with your hands). Will we be able to get a tight enough fit? Any other concerns we should think about? Thanks!!